Bearing Bush and Compound Movement Using the Same

ABSTRACT

Provided is a bearing bush enabling excellent advancing and retreating movements of a shaft member by eliminating a clearance between the bearing bush and the shaft member to eliminate a backlash therebetween, capable of reducing a load of dimension control on both the bearing bush and the shaft member, and capable of being manufactured at a low cost. The bearing bush ( 4 ) for supporting the reciprocating movement of the shaft member ( 1 ) in an axial direction has a receiving hole through which the shaft member ( 1 ) is passed. Grooves ( 40 ) are sequentially formed at predetermined intervals in an inner peripheral surface of the bearing bush ( 4 ) facing the receiving hole. An inner diameter of the receiving hole is formed to be equal to or smaller than an outer diameter of the shaft member ( 1 ), and the bearing bush is press-fitted to an outer peripheral surface of the shaft member ( 1 ) in a state of a so-called interference fit.

TECHNICAL FIELD

The present invention relates to a bearing bush for supporting axialadvancing and retreating movements of a shaft member, such as a splineshaft. For example, the present invention relates to a bearing bush usedto support advancing and retreating movements of a spline shaft of acompound movement device having the spline shaft as the main shaft andcapable of imparting a rotating motion to the spline shaft while causingthe spline shaft to advance and retreat.

BACKGROUND ART

A main shaft of an automatic work supply device or of an automatic toolreplacing device in an NC machine tool is required to be capable ofmaking a rotational movement with high accuracy while freely advancingor retreating in the axial direction. In this connection, a combinationof a spline shaft and a spline nut is used as means for supporting anaxial movement of the main shaft and transmitting rotational torque tothe main shaft.

In particular, in a ball spline, in which a spline nut is assembled to aspline shaft through the intermediation of a large number of balls,preload is imparted to the balls, whereby, when transmission of therotational torque is effected between the spline nut and the splineshaft, no backlash is generated between the two components, making itpossible to cause the spline shaft to advance and retreat smoothly whiletransmitting the rotational torque thereto.

A known conventional example of a compound movement device using suchthe ball spline is composed of a spline shaft having a plurality of ballrolling grooves extending in the axial direction, a spline nut assembledto this spline shaft through the intermediation of a large number ofballs and capable of reciprocating in the axial direction, asubstantially cylindrical housing which accommodates the spline nut andthrough which the spline shaft passes, and a rotation transmittingmember, such as a pulley or a gear, mounted to the housing.

In providing the rotation transmitting member on the housing, if therotation transmitting member is superimposed on the spline nut, theouter diameter of the rotation transmitting member becomes rather large,resulting in an increase in the device size and an increase in cost. Inview of this, there is employed, as the housing, a stepped cylindricalmember in which a large diameter portion and a small diameter portionare continuous with each other, with the large diameter portion beingused as an accommodating portion for the spline nut, and the smalldiameter portion being used as a mounting portion for the rotationtransmitting member.

On the other hand, when such the stepped cylindrical member is used asthe housing and the spline nut and the rotation transmitting member aredeviated from each other in the axial direction of the housing, a radialload applied to the housing from the rotation transmitting member (i.e.,load perpendicular to the axial direction of the spline shaft) isallowed to act on the spline nut as a moment load, and if there is nosupport at all for the forward end of the small diameter portion of thehousing with respect to the spline shaft, there is a disadvantage inthat the housing will incline with respect to the spline shaft.

In view of this, a bearing bush formed of synthetic resin is arrangedbetween the forward end of the small diameter portion of the housing andthe spline shaft to thereby reduce the moment load acting on the splinenut. While this bearing bush is forced into the inner peripheral surfaceof the small diameter portion of the housing with pressure, to suppressan increase in the sliding resistance of the housing with respect to thespline shaft, the bearing bush is fitted onto the spline shaft in astate of a so-called clearance fit through the intermediation of aminute clearance (e.g., approximately 0 to 24 μm when the outer diameterof the spline shaft is 6 mm) with respect to the spline nut.

However, when the resin bearing bush fixed to the small diameter portionof the housing is thus fitted onto the spline shaft in a state of aclearance fit, due to the presence of a clearance, although extremelysmall, between the spline shaft and the bearing bush, the contactbetween the bearing bush and the spline shaft becomes rather unstableand the sliding resistance of the housing with respect to the splineshaft undergoes fluctuation or the like, so there arises a problem inthat the precision in the axial advancing/retreating movement of thehousing is likely to be impaired.

Further, strict control of the inner diameter dimension of the bearingbush and the outer diameter dimension of the spline shaft is required,which leads to an increase in production cost.

As disclosed in JP 2001-12472 A, in a known conventional guide devicefor a shaft member using a bearing bush formed of synthetic resin, thebearing bush and the shaft member are held in press contact with eachother in a state of a so-called interference fit. In this guide device,the interference with which the two members are fit-engaged with eachother is controlled by performing dimension control on the innerdiameter of the bearing bush and the outer diameter of the shaft member,whereby the sliding resistance of the bearing bush with respect to theshaft member is kept within a predetermined range.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, to completely eliminate the clearance between the bearing bushand the shaft member without involving an increase in slidingresistance, it is still necessary to perform strict dimension control onthe inner diameter of the bearing bush and the outer diameter of theshaft member, thereby requiring a lot of time and effort for production.

The present invention has been made in view of the above-mentionedproblems. Therefore, it is an object of the present invention to providea bearing bush in which the clearance between the bearing bush and theshaft member is eliminated to thereby eliminate backlash therebetween,enabling the shaft member to perform advancing/retreating movements in asatisfactory manner, and in which the burden of dimension control forboth the bearing bush and the shaft member is lessened, thus achieving areduction in production cost.

Means for Solving the Problems

That is, according to the present invention, there is provided a bearingbush for supporting axial reciprocating movement of a shaft member, inwhich the bearing bush has a receiving hole through which the shaftmember is passed, with grooves being sequentially formed atpredetermined intervals in an inner peripheral surface facing thereceiving hole. The inner peripheral surface of the bearing bush formsno clearance between itself and the shaft member, and is in presscontact with the shaft member.

According to the present invention, grooves are formed sequentially atpredetermined intervals in the inner peripheral surface of the bearingbush, so even if the bearing bush is fit-engaged with the shaft memberby interference fit, the sliding contact surface of the bearing bush inpress contact with the shaft member is crushed relatively easily, makingit possible to reduce the press contact force with respect to the shaftmember. Further, it is also possible to reduce the contact area betweenthe bearing bush and the shaft member.

Thus, it is possible to minimize the sliding resistance between thebearing bush and the shaft member while eliminating the clearancetherebetween, making it possible to achieve enhancement in the accuracyof the advancing/retreating movements of the shaft member with respectto the bearing bush. Further, due to the formation of a plurality ofgrooves in the inner peripheral surface of the bearing bush, the slidingcontact surface of the bearing bush in press contact with the shaftmember is crushed and deformed relatively easily, so it is possible tominimize the sliding resistance of the bearing bush with respect to theshaft member without having to perform strict control on the innerdiameter dimension of the bearing bush, also making it possible toproduce the bearing bush at so much the lower cost.

The bearing bush used in the present invention may be formed of metal orsynthetic resin. However, from the viewpoint of reducing the presscontact force generated when fit-engaging the bearing bush and thespline shaft with each other in a state of an interference fit,synthetic resin is more preferable.

The bearing bush of the present invention can be used in theabove-described compound movement device. In the compound movementdevice, the advancement/retreating of the spline shaft is supported bythe large number of balls with which the spline nut is provided. Whenthe spline shaft advances or retreats at high acceleration ordeceleration, the vibration due to the rolling of the balls acts on thespline shaft. However, in the present invention, when theadvancing/retreating movements of the spline shaft are supported byusing the bearing bush, the bearing bush is in press contact with thespline shaft, so the attenuation effect with respect to the vibration ofthe spline shaft is improved. Thus, even when the spline shaft advancesor retreats at high acceleration or deceleration, the requisitepositioning time for the spline shaft tends to be shorter. As a result,it is possible to shorten the tact time of an operation using thiscompound movement device, thus making it possible to produce machinesand apparatuses with satisfactory productivity.

Further, the bearing bush of the present invention can be used forsupporting the advancing/retreating movements of an output rod of alinear motor actuator. A known example of a linear motor actuator iscomposed of a magnet rod on which a large number of magnetic poles arearranged and a coil member loosely fitted around the magnet rod andadapted to drive the magnet rod in the axial direction. When this magnetrod is used as the output rod, it is necessary to support the advancingand retreating movements of the magnet rod by a bearing member. When thebearing bush of the present invention is used as the bearing member, itis possible to eliminate the clearance between the bearing bush and themagnet rod and to minimize the sliding resistance between the twomembers, making it possible to enhance the accuracy of the advancing andretreating movements of the magnet rod with respect to the bearing bush.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A front sectional view of an embodiment of a compound movementdevice using a bearing bush according to the present invention.

FIG. 2 A perspective view of an example of a ball spline that can beused in the compound movement device according to the present invention.

FIG. 3 An enlarged view showing how the bearing bush and a spline shaftare held in contact with each other.

FIG. 4 A side view of the bearing bush.

FIG. 5 A side view of another example of a bearing bush.

FIG. 6 An enlarged view of an example in which a lubricant supply memberis provided adjacent to the bearing bush.

FIG. 7 A front sectional view of an embodiment of a linear motoractuator using the bearing bush according to the present invention.

FIG. 8 A side view illustrating an operation principle of a linear motoraccording to an embodiment.

FIG. 9 A front view illustrating the operation principle of the linearmotor according to the embodiment.

DESCRIPTION OF REFERENCE NUMERALS

1 . . . spline shaft, 2 . . . spline nut, 3 . . . housing, 4 . . .bearing bush, 40 . . . groove, 41 . . . protrusion

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, a bearing bush according to the present invention willbe described in detail with reference to the accompanying drawings.

FIG. 1 shows an example of a compound movement device supportingadvancing and retreating movements of a spline shaft by using a bearingbush according to the present invention. This device is used as the mainshaft of a tool replacement device or the like, and is composed of aspline shaft 1 used as the main shaft, a spline nut 2 capable ofreciprocating in the axial direction of the spline shaft 1, a housing 3for retaining the spline nut 2 and supported so as to be rotatable withrespect to a stationary portion B, and a bearing bush 4 provided at oneend of the housing 3, for guiding the advancing and retreating of thespline shaft 1 with respect to the housing 3.

The spline shaft 1 is caused to make axial advancing and retreatingmovements by a ball screw device, a hydraulic cylinder, a pneumaticcylinder, or the like (all of which are not shown), and the advancingand retreating movements are supported by the spline nut 2. Fixed to thehousing 3 is a rotation transmitting member 5, such as a pulley or agear, around which a timing belt is wrapped, rotational torque isimparted to the housing 3 from the outside, and this rotational torqueis transmitted to the spline shaft 1 through the spline nut 2, making itpossible to arbitrarily rotate the spline shaft 1.

FIG. 2 shows an example of a ball spline in which the spline shaft 1 andthe spline nut 2 are combined with each other. The spline shaft 1 has asubstantially circular sectional configuration, and has on the outerperipheral surface thereof a plurality of ball rolling grooves 10extending in the axial direction. The spline nut 2 has a substantiallycylindrical configuration and has a through-hole through which thespline shaft 1 is passed. The spline nut 2 is assembled to the splineshaft 1 through the intermediation of a large number of balls 20 rollingin the ball rolling grooves 10 of the spline shaft 1. Further, thespline nut 2 is equipped with an endless circulation path 21 throughwhich the balls 20 circulate, and can continuously move along the splineshaft 1 while circulating the balls 20 in the endless circulation path21. Preload is imparted to the balls 20 rolling between the spline shaft1 and the spline nut 2 while receiving a load, thereby eliminatingbacklash between the spline nut 2 and the spline shaft 1. Even whenrotational torque is being transmitted to the spline shaft 1 from thespline nut 2, it is possible to cause the spline shaft 1 to advance andretreat in the axial direction smoothly and accurately.

The housing 3 is substantially formed as a cylinder covering theperiphery of the spline shaft 1, and is equipped with a large diameterportion 30 accommodating the spline nut 2 and a small diameter portion31 whose outer diameter is smaller than that of the large diameterportion 30, with the housing as a whole being formed as a steppedcylinder. A rotary bearing (not shown) is mounted to the outerperipheral surface of the large diameter portion 30, whereby the housing3 is supported so as to be rotatable with respect to the stationaryportion B. Further, the spline nut 2 is fit-engaged with the innerdiameter side of the large diameter portion 30, and is fixed to thelarge diameter portion 30 by a bolt radially extending through the largediameter portion 30.

The small diameter portion 31 is formed to exhibit an outer diametersmaller than that of the large diameter portion 30, and a rotationtransmitting member 5, such as a pulley, is fixed to the outerperipheral surface thereof. The inner diameter of the small diameterportion 31 is slightly larger than the outer diameter of the splineshaft 1, enabling the spline shaft 1 to freely advance and retreat inthe axial direction within the small diameter portion 31.

The bearing bush 4 is mounted to one end of the small diameter portion31 side of the housing 3, that is, to the forward end of the smalldiameter portion 31, guiding the advancing and retreating movements ofthe spline shaft 1 with respect to the housing 3. The bearing bush 4 isformed of a synthetic resin (e.g., polyacetal) which is superior infrictional wear resistance and is self-lubricant, and which is endowedwith high elasticity modulus. As shown in FIG. 3, the bearing bush 4 isprovided between the inner peripheral surface of the small diameterportion 31 of the housing 3 and the outer peripheral surface of thespline shaft 2. The bearing bush 4 has a substantially cylindricalconfiguration with a receiving hole through which the spline shaft 1 ispassed, and is press-fitted into the inner peripheral surface of thesmall diameter portion 31 of the housing 3 and fixed in positiontherein. The inner diameter of the receiving hole is equal to or smallerthan the outer diameter of the spline shaft 1, and the receiving hole isin press contact with the outer peripheral surface of the spline shaft 1in a so-called interference fit state. In this interference fit,assuming, for example, that the interference is γ and the shaft diameteris D, γ is approximately equal to D/1000.

As shown in FIGS. 3 and 4, a plurality of circumferential grooves 40 areformed at predetermined intervals in the inner peripheral surface of thebearing bush 4, with the portions of the inner peripheral surface otherthan the grooves 40 being defined as annular protrusions 41 whose distalends are in press contact with the outer peripheral surface of thespline shaft 1. Thus, if the bearing bush 4 is fit-engaged with thespline shaft 1 in a state of interference fit, the distal ends of theprotrusions 41 formed in the inner peripheral surface of the bearingbush 4 are crushed by the spline shaft 1 and easily undergo elasticdeformation, thus reducing the press contact force between the bearingbush 4 and the spline shaft 2. As a result, even if the bearing bush 4is fit-engaged with the spline shaft 1 in a state of interference fit,it is possible to reduce the sliding resistance exerted therebetween.Further, due to the formation of a plurality of grooves in the innerperipheral surface of the bearing bush 4, the contact area between thespline shaft 1 and the bearing bush 4 becomes smaller than that in thecase in which no grooves 40 are formed, which also helps to reduce thesliding resistance between the bearing bush 4 and the spline shaft 1.

Further, with this construction, in which a plurality of circumferentialgrooves 40 are formed in the inner peripheral surface of the bearingbush 4 and in which a plurality of protrusions 41 between the grooves 40are held in press contact with the outer peripheral surface of thespline shaft 1, the protrusions 41 repeatedly scrape the outerperipheral surface of the advancing and retreating spline shaft 1, sothe bearing bush functions as a seal device, making it possible toprevent dust on the spline shaft 1 from entering the housing 3.

As stated above, the bearing bush 4 may be formed of a synthetic resinhaving a self-lubricant property. However, it is also possible topreviously fill the grooves 40 with grease so that the grease in thegrooves 40 may be applied to the outer peripheral surface of the splineshaft 1 as the spline shaft 1 advances and retreats. By thus filling thegrooves of the bearing bush with grease, it is possible to furtherimprove the function of the bearing bush as a seal device.

In the compound movement device constructed as described above, whenrotational torque is transmitted to the pulley 5 fixed to the smalldiameter portion 31 of the housing 3 through, for example, a timing belt50, it is possible to rotate the housing 3 rotatably supported withrespect to the stationary portion B. Thus, it is possible to transmitrotational torque to the spline shaft 1 supported by the housing 3through the intermediation of the spline nut 2, making it possible toarbitrarily rotate the spline shaft 1 around the axis thereof. Further,in the state in which the spline shaft 1 is thus being rotated, it ispossible to arbitrarily cause the spline shaft 1 to advance and retreatin the axial direction.

Since the small diameter portion 31 of the housing 3 is provided withthe bearing bush 4 for guiding the spline shaft 1, and the bearing shaft4 is fit-engaged with the spline shaft 1 by interference fit, theclearance between the two members is eliminated, making it possible tosmoothly guide the spline shaft 1 with constantly stable slidingresistance.

Further, while the spline shaft 1 and the bearing bush 4 are in presscontact with each other by interference fit, due to the provision of aplurality of grooves 40 in the inner peripheral surface of the bearingbush 4, it is possible, as stated above, to reduce the press contactforce between the spline shaft 1 and the bearing bush 4, and further, toreduce the contact area, so that it is also possible to minimize thesliding resistance of the spline shaft 1 with respect to the bearingbush 4.

Further, as a result of the provision of a plurality of grooves 40 inthe inner peripheral surface of the bearing bush 4, the distal ends ofthe protrusions 41 in contact with the spline shaft 1 are crushedrelatively easily and undergo deformation, so in fit-engaging thebearing bush 4 with the spline shaft 1 by interference fit, it ispossible to minimize the sliding resistance of the bearing bush 4 withrespect to the spline shaft 1 without having to perform a very strictcontrol on the inner diameter dimension of the bearing bush 4. As aresult, the requisite time and effort for dimension control in theproduction of the bearing bush 4 are reduced, making it possible toproduce the device at so much the lower cost.

A bearing bush having an axial length of 3 mm, an outer diameter of 8mm, and an inner diameter of 6 mm (tolerance: 0 to −0.010 mm) wasactually fit-engaged with a spline shaft having an outer diameter of 6mm (tolerance: 0 to −0.012 mm) to examine the backlash of the splineshaft with respect to the bearing bush and the increase in slidingresistance. Four grooves having a width of 0.75 mm and a depth of 0.4 to0.7 mm were formed in the inner peripheral surface of the bearing bush.The examination result showed that there was no backlash of the splineshaft with respect to the bearing bush, and the spline shaft could beguided smoothly. As for the increase in sliding resistance, it wassufficiently allowable in terms of practical use.

While in the bearing bush 4 shown in FIG. 3, the grooves 40 have arectangular sectional configuration and the protrusions 41 are alsorectangular, from the viewpoint of making the distal ends of theprotrusions 41 easier to deform elastically, it is desirable for thegrooves 40 to be formed in a triangular sectional configuration and forthe distal ends of the protrusions 41 to be formed in a steeple-likeconfiguration.

While in the bearing bush 4 shown in FIGS. 3 and 4, a plurality ofgrooves 40 are formed in the inner peripheral surface thereof so as tobe parallel to each other, it is also possible for the grooves 40 to beformed as a single or a plurality of continuous spiral grooves.

FIG. 5 shows another example of the bearing bush 4. While in the bearingbush 4 shown in FIGS. 3 and 4, circumferential grooves 40 are formed inthe inner peripheral surface thereof, in a bearing bush 6 shown in FIG.5, a plurality of axial grooves 60 are formed, and ridges 61 definedbetween these grooves are held in press contact with the outerperipheral surface of the spline shaft 1 by interference fit. With thebearing bush 6 also, the same effect as that of the above-describedbearing bush 4 can be obtained.

FIG. 6 shows an example in which a lubricant supply member 7 is providedinside the small diameter portion 31 of the housing 3. The supply member7 is composed of a ring member 70 adjacent to the bearing bush 4 andfit-engaged with the inner peripheral surface of the small diameterportion 31 of the housing 3, and an application member 71 fixed to theinner peripheral surface of the ring member 70 and consisting of felt orthe like impregnated with lubricant. The application member 71 is heldin contact with the outer peripheral surface of the spline shaft 1, andwhen the spline shaft 1 advances or retreats, the application member 71applies a requisite minimum amount of lubricant to the outer peripheralsurface thereof. As stated above, the bearing bush 4 of the presentinvention also functions as a seal device for sealing the clearancebetween the small diameter portion 31 of the housing 3 and the splineshaft 1. Thus, when the lubricant supply member 7 is provided inside thesmall diameter portion 31 so as to be adjacent to the bearing bush 4,there is no fear of the lubricant applied to the spline shaft 1 flowingout. Further, the lubricant is applied to the outer peripheral surfaceof the spline shaft 1, from which fine dust has been scraped away by thebearing bush 4, so it is possible to achieve further improvement interms of lubrication performance.

FIG. 7 shows an example in which the bearing bush of the presentinvention is used to support the output rod of a linear motor actuator8. The linear motor actuator 8 is composed of a housing 81 in which athrough-hole 80 is formed, an output rod 82 which is provided so as toextend through the through-hole 81 and on which a large number ofmagnetic poles are arranged at a predetermined pitch in an axialdirection, a coil member 83 which is loosely fitted onto the peripheryof the output rod 82 and fixed to the housing 81, for driving the outputrod 82 in the axial direction, and a pair of bearing members 84, 84fixed to the housing 81 at the openings at the ends of the through-holeand supporting advancing and retreating movements of the output rod.This linear motor actuator can cause the output rod 82 to freely advanceand retreat in the axial direction and to stop at an arbitrary position.

The output rod 82 and the coil member 83 constitute a linear motor, andthe coil member 83 is loosely fitted onto the periphery of the outputrod 82 with a slight gap therebetween. The output rod 82 is magnetizedso as to exhibit a plurality of permanent magnets which are arranged inthe axial direction, with the outer peripheral surface of the output rodbeing machined into a smooth surface. As shown in FIG. 8, each permanentmagnet 84 has an N-pole and an S-pole, and the adjacent permanentmagnets 84 are arranged with their orientations being alternatelyreversed so that the permanent magnets of the same polarity are opposedto each other. As a result, the output rod 82 has a magnetized portionfor driving, in which the N-poles and the S-poles are alternatelyarranged in the longitudinal direction to form a field magnet.

FIGS. 8 and 9 show the operation principle of this linear motor. Thecoil member 83 has a coil group in which three excitation coils 86 ofU-, V-, and W-phase constitute one set. The excitation coils 86 of allthe phases have a ring-like configuration, and are opposed to the outerperipheral surface of the output rod 82 with a slight gap therebetween.The arrangement pitch of the excitation coils 86 of the different phasesis set to be smaller than the arrangement pitch of the permanent magnets84. Magnetic fluxes 85 are formed on the output rod 82 from the S-polestoward the N-poles, and the coil member 83 contains a magnetic polesensor (not shown) for detecting the magnetic flux density thereof.Thus, the positional relationship of the magnetic poles (i.e., N-polesand S-poles) of the output rod 82 with respect to the excitation coils86 is grasped from a detection signal output from this magnetic polesensor. A controller controlling the supply of electricity to theexcitation coils 86 receives the detection signal from the magnetic polesensor and calculates an optimum electric current corresponding to thepositional relationship between the excitation coils 86 and the magneticpoles of the output rod 82, thereby supplying the electric current tothe excitation coils 86. As a result, due to the interaction between theelectric current flowing through the excitation coils 86 and themagnetic fluxes 85 formed by the permanent magnets 84, an attractionforce and a repellent force are generated between the excitation coils86 and the magnetic poles of the permanent magnets 84, and the outputrod 82 is propelled in the axial direction with respect to the coilmember 83 fixed to the housing 81.

A housing 61 containing the coil member is formed of aluminum, which issuperior in heat conductivity. From the viewpoint of efficientlyconducting to the housing 81 the heat generated in the excitation coils86 when the excitation coils 86 are energized and radiating the heatinto the ambient atmosphere, and from the viewpoint of effectivelycooling the excitation coils 86 themselves, it is desirable to form aplurality of radiation fins on the surface of the housing 81.

As each of the pair of bearing members 84 supporting the output rod 82with respect to the housing 81, the bearing bush of the presentinvention used in the compound movement device described above isapplicable as it is.

That is, when the bearing bush 4 of the present invention shown in FIG.4 is used as each of the bearing members 84, the bearing bushes 4 arefit-engaged with the output rod 82 by interference fit, so the gaptherebetween is eliminated, making it possible to smoothly guide theoutput rod 82 with constantly stable sliding resistance.

Further, while the output rod 82 is held in press contact with thebearing bushes 4 by interference fit, due to the provision of aplurality of grooves 40 in the inner peripheral surfaces of the bearingbushes 4, the press contact force between the output rod 82 and thebearing bushes 4 is reduced as stated above. Further, since the contactarea can be reduced, it is possible to minimize the sliding resistanceof the output rod 82 with respect to the bearing bushes 4.

Further, as a result of the provision of a plurality of grooves 40 inthe inner peripheral surfaces of the bearing bushes 4, the distal endsof the protrusions 41 in contact with the output rod 82 are crushedrelatively easily and undergo deformation, so in fit-engaging thebearing bushes 4 with the output rod 82 by interference fit, it ispossible to minimize the sliding resistance of the bearing bushes 4 withrespect to the output rod 82 without having to perform a very strictcontrol on the inner diameter dimension of the bearing bushes 4. As aresult, the requisite time and effort for dimension control in theproduction of the bearing bushes 4 are reduced, making it possible toproduce the linear motor actuator at so much the lower cost.

1. A bearing bush for supporting an axial reciprocating movement of ashaft member, characterized in that the bearing bush has a receivinghole through which the shaft member is passed, and grooves are formedsequentially at predetermined intervals in an inner peripheral surfacefacing the receiving hole, with the inner peripheral surface being inpress contact with the shaft member.
 2. The bearing bush according toclaim 1, characterized in that the grooves in the inner peripheralsurface are formed circumferentially.
 3. The bearing bush according toclaim 1, characterized in that the grooves in the inner peripheralsurface are formed spirally.
 4. The bearing bush according to claim 1,characterized in that the bearing bush is formed of synthetic resin. 5.A compound movement device, characterized in that the compound movementdevice comprises: a spline shaft having a plurality of ball rollinggrooves extending in an axial direction; a spline nut assembled to thespline shaft through an intermediation of a large number of balls andcapable of reciprocating in the axial direction; a housing formed as astepped cylindrical member having a large diameter portion and a smalldiameter portion that are continuous with each other, with the splineshaft extending through the large diameter portion and the smalldiameter portion, the large diameter portion being used as anaccommodating portion for the spline nut, and an outer peripheralsurface of the small diameter portion being used as a mounting surfacefor a rotation transmitting member; and a bearing member fixed to thesmall diameter portion of the housing, for supporting advancement andretreating of the spline shaft, and in that the bearing member is thebearing bush according to claim
 1. 6. The compound movement deviceaccording to claim 5, characterized in that the bearing bush is forcedinto the small diameter portion of the housing.
 7. The compound movementdevice according to claim 5, characterized in that the grooves in aninner peripheral surface of the bearing bush are filled with grease. 8.The compound movement device according to claim 5, characterized in thata lubricant supply member for applying lubricant to an outer peripheralsurface of the spline shaft is provided inside the small diameterportion of the housing at a position on an inner side of the bearingbush so as to be adjacent to the bearing bush.
 9. A linear motoractuator, characterized in that the linear motor actuator comprises: ahousing; an output rod provided so as to be capable of advancing andretreating with respect to the housing and having a large number ofmagnetic poles arranged at a predetermined pitch in an axial direction;a coil member loosely fitted onto an outer periphery of the output rodand fixed to the housing, for driving the output rod in the axialdirection; and a bearing member fixed to the housing, for supporting theadvancing and retreating of the output rod, and in that the bearingmember is the bearing bush according to claim 1.